Color video signals, so-called composite video, blanking and sync signals (CVBS;) are essentially composed of a brightness signal or luminance component (Y), two color difference signals or chrominance components (U, V or I, Q), vertical and horizontal sync signals (VS, HS) and a blanking signal (BL). The structure of a composite video signal (CVBS) and the corresponding Y, U and V signals is shown in FIGS. 1a and 1b.
FIG. 1a shows a composite video signal for an EBU (European Broadcasting Union) color bar test signal, in which the six hue (tint) values belonging to the vertical color bar in "carrier packets" with color carrier frequency are superimposed on the luminance component Y. For the color carrier generation a color subcarrier frequency sync signal, the burst, is transmitted directly behind the line sync pulse, SYNC. The burst phase and the burst amplitude are used as reference values for determining the hue and the color saturation of the demodulated signal, which is represented by the individual carrier packets.
The different coding processes, e.g. NTSC, PAL and SECAM, introduced into the known color television standards, differ in the nature of the chrominance transmission and in particular the different systems make use of different color subcarrier frequencies and different line frequencies.
The following explanations relate to the PAL and NTSC systems, but correspondingly apply to video signals of other standards and non-standardized signals.
The color subcarrier frequency (fsc) of a PAL system and a NTSC system is EQU fsc(NTSC)=3.58 MHz or EQU fsc(PAL)=4.43 MHz.
In addition, in PAL and NTSC systems the relationships of the color subcarrier frequency (fsc) to the line frequency (fh) are given by EQU fsc(NTSC)=227.50*fh or 4.multidot.fsc(NTSC)=910.multidot.fh EQU fsc(PAL)=283.75*fh or 4.multidot.fsc(PAL)=1135.multidot.fh
so that the phase of the color subcarrier in the case of NTSC is changed by 180.degree./line and in PAL by 270.degree./line.
In the case of digital video signal processing and decoding the prior art fundamentally distinguishes between two system architectures. These are the burst-locked architecture and the line-locked architecture. These systems operate with sampling frequencies for the video signal, which are produced in phase-locked manner to the color subcarrier frequency transmitted with the burst pulse or in phase-locked manner with the line frequency, respectively.
In the case of decoders with a burst-locked architecture the sampling frequency is chosen in such a way that on the one hand it is not too high so as to keep the power loss low and on the other hand so that the Nyquist theorem is fulfilled. i.e. f.sub.a &gt;2.multidot.fsc. For problem free processing of the modulated color carrier in the decoder it is appropriate to use a sampling frequency which corresponds to the even multiple of the color subcarrier. Four times the color subcarrier frequency is preferred.
In the case of line-locked architectures the clock of the digital system is derived from the line frequency and is an integer multiple of the line frequency, so that an integer number of pixels per line are produced. Although the burst-locked system has advantages with respect to the minimum effort and expenditure for color decoding, but it also has disadvantages, in the horizontal and vertical synchronization, as well as in multistandard and "non-standard" uses. As the sampling process is essentially non-orthogonal, the burst-locked system is only suitable for the direct representation of images on a screen, but not for producing data for fixed raster applications, e.g., for field or frame stores or for frame grabbers in a PC environment.
Although the line-locked systems solve the problems of burst-locked architectures in a satisfactory manner, they still lead to new problems. In particular, they lead to much more complex color decoders, a high circuitry expenditure for the analog clock generation and higher requirements for the maximum acceptable dynamic non-linearities of the A/D converters and preceding analog signal processing stages.
Both systems suffer from the disadvantages that the clock frequencies for digitizing the video signal are derived from the video system, namely from the color subcarrier frequency or from the line frequency. Whereas in a PC environment, working takes place with completely different clock frequencies. The different clock frequencies in the overall system intermodulation products and causes crosstalk of signals which has a disturbing effect on the overall operation and the image quality. As the clock frequencies of PCs are not generally suitable for sampling video signals, and because they do not satisfy the above-explained conditions, the prior art decoders in each case have their own oscillators for producing the sampling frequency suitable for a particular television standard.
In summary, Table I gives different sampling frequencies and their advantages/disadvantages for digital video processing:
TABLE I ______________________________________ Sampling Frequency Advantages Disadvantages ______________________________________ Burst-locked Simple color decoder. Complicated line (4.multidot.f.sub.sc) No distortion due to synchronization. A non-linearities. sample rate conversion is necessary for field/frame storages. For multistandard operation multiple crystals are necessary. Simple field/frame storage. Line-locked Only one crystal for Complicated color (n.multidot.f.sub.H) multistandard operation. decoder. Sensitive to non-linearities in composite video signal. ______________________________________
The best choice with respect to these factors is dependent on the market, the function and intended use of the decoder. A rough survey is given in Table II.
TABLE II ______________________________________ Preferred Market Use Sampling ______________________________________ TV set Replacement of analog Burst-locked circuit. TV set Additional features Line-locked (field/frame store) Desk top video (PC) Fixed video image size in Burst-locked a window. Desk top video (PC) Scalable video image size Line-locked in a window. ______________________________________